The invention relates to a method for stacking spent nuclear fuel assemblies in a spent fuel pool and more particularly to a method for stacking spent fuel assemblies in two tiers.
Pressurized water nuclear plants and boiling water nuclear plants for commercially generating electricity are refueled on routinely scheduled outages every one to two years. In the course of a refueling outage, a reactor vessel cavity and adjoining refueling canal are submerged in a pool of water and spent nuclear fuel assemblies are removed from a reactor vessel by a refueling machine and transferred by other devices under water to large spent fuel pools connected with the refueling canal by a transfer tube. The removed spent fuel assemblies then are stored in spent fuel storage racks located at the bottom of the spent fuel pool until such time as they can be safely removed from the generating plants to be processed or permanently stored elsewhere. See, e.g., U.S. Pat. Nos. 5,245,641; 4,960,560; 4,900,506; 4,746,487; 4,400,344; 4,233,518 and 4,010,375.
If remote spent fuel assembly processing facilities and permanent storage facilities can not accept shipments of spent fuel assemblies, the generating plants may need to retain the spent fuel assemblies on site in the spent fuel pools for longer periods of time than was contemplated when the spent fuel pools were designed. Accordingly, the generating plants have had to modify their storage equipment and their storage practices in order to increase the capacities of their spent fuel pools.
In addition, it has been proposed in the industry to stack the spent fuel assemblies in a second tier of spent fuel storage racks over a first tier of spent fuel storage racks. See, e.g., U.S. Pat. Nos. 5,152,958; 4,889,681 and 4,029,968. However, these proposals generally require extensive modifications of the pools or the use of handling equipment which is either not practical or not cost effective.
It is an object of the present invention to provide an improved method for increasing the number of fuel assemblies that can be stored in existing spent fuel pools. It is a further object to provide a practical method for more readily handling additional spent fuel assemblies in the existing pools.
With these objects in view, the present invention resides in an improved method of stacking spent nuclear fuel assemblies in a spent nuclear fuel assembly pool containing previously installed first nuclear fuel assembly storage racks. In accordance with the invention, a tier of second nuclear fuel assembly storage racks is installed in the spent nuclear fuel assembly pool over and structurally supported independently of structural support members of the previously installed spent nuclear fuel assembly storage racks. One or more vertically oriented spent fuel assemblies are then introduced into the second fuel assembly storage while vertically oriented.
In a preferred practice of the present invention, a spent nuclear fuel assembly is raised at least partially out of the spent nuclear fuel assembly pool before introducing the spent nuclear fuel assembly into the upper tier second nuclear fuel assembly storage racks. In some practices, the spent nuclear fuel assembly been stored in the previously installed first spent nuclear fuel assembly storage rack before transferring it to the upper tier second nuclear fuel assembly storage rack. Advantageously, fuel assemblies that have cooled in a lower tier first storage rack for a number of years may be safely stored in upper tier second nuclear fuel assembly storage racks because they require substantially less shielding. In addition, the nuclear fuel assembly storage racks in the upper tier also absorb radiation emitted by the lower tier first spent nuclear fuel assembly storage racks. This technique of using older, colder spent nuclear fuel assemblies to shield newer, hotter spent nuclear fuel assembly is known in the industry as xe2x80x9czone loadingxe2x80x9d.